A vacuum circuit breaker includes a stationary contact point and a moving contact point that is placed opposite the stationary contact point. The stationary contact point and the moving contact point are placed in a vacuum chamber in which vacuum is maintained around the contact points. With the contact points of the vacuum circuit breaker closed to cause current to flow in a main circuit conductor, when the contact points of the vacuum circuit breaker are opened to interrupt current flowing in the main circuit, if the vacuum degree in the vacuum chamber is high, high arc-suppression capability of vacuum causes current to be interrupted. However, if the vacuum degree in the vacuum chamber decreases due to factors such as a crack in the vacuum chamber, a discharge of gas molecule adsorbed to a metal or insulator, as well as a transmission of atmospheric gas, opening the contact points may cause breakdown to prevent the interruption of current, which may damage the equipment in the worst case. Thus, in order to know what is going on in the vacuum circuit breaker for preventing the breaker and its peripheral devices from being damaged, a vacuum degree deterioration monitoring device for determining whether or not the vacuum degree in the vacuum chamber has deteriorated has been under development.
For example, in existing literatures, a method is disclosed in which the normality of the vacuum degree is determined by detecting, using an antenna mounted in the tank-type circuit breaker, an electromagnetic wave caused by a discharge due to vacuum degree deterioration resulting in withstand voltage reduction (PTL 1). Furthermore, in one discharge detection method, a metal electrode placed in a bushing is used as an antenna to enable detection of a wide-range frequency signal (PTL 2). Furthermore, a method is described in which a discharge location is detected using the time difference of a signal received by two or more antennas mounted in the breaker (PTL 3).
Furthermore, a method is described in which a conductor is placed in a bushing to receive a discharge signal (PTL 4).